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  1. State that the Sun is the principal source of energy input to biological systems.

The Sun is the principal source of energy input to biological systems.

If you’re having trouble understanding why, visualise a food web. Carnivores gain their energy from consuming other animals, Herbivores gain their energy from consuming plants, and plants gain their energy from the Sun.

Therefore, the Sun is the ultimate source of energy in all biological systems.


  1. Define the terms:
  • Food chain as the flow of energy (food) from on organism to the next beginning with a producer (e.g. mahogany tree -> caterpillar -> song bird -> hawk)
  • Food web as a network of interconnected food chains showing the energy flow through part of an ecosystem
  • Producer as an organism that makes its own organic nutrients, usually using energy from sunlight, through photosynthesis
  • Consumer as an organism that gets its energy by feeding on other organisms
  • Herbivore as an animal that gets its energy by eating plants
  • Carnivore as an animal that gets its energy by eating other animals

Definitions are important – learn ‘em.


  1. Describe energy losses between trophic levels.

In any self-respecting food chain, the first item is the Sun. Then comes the producer (the first trophic level), then the primary consumer (the second trophic level), the secondary consumer (third trophic level), sometimes there’s a tertiary consumer (fourth trophic level), and even more rarely, there’s a quaternary consumer (the fifth trophic level).

We all know that the Sun has an amazing amount of energy.

The proportion of that energy that arrives at Earth is very small.

Of all the energy that does reach Earth, slightly less than 34% is reflected back to space by clouds.

The Earth itself reflects another 66% back to space.

Less than one percent of the total energy that reaches Earth is used by plants for photosynthesis.

From here on, around 10% or less is passed to the next trophic level.

The rest is lost as heat, sound, in metabolism, used for movement, in faeces, etc.


  1. Define the terms:
  • Decomposer as an organism that gets its energy from dead or waste organic matter
  • Ecosystem as a unit containing all of the organisms and their environment, interacting together, in a given area, e.g. decomposing log or a lake
  • Trophic level as the position of an organism in a food chain or food web.

Hear ye, Hear ye! Yet more definitions! You guys should know the drill by now.


  1. Explain why food chains usually have fewer than five trophic levels.

As explained earlier, a lot of energy is lost between trophic levels.

The most energy is available for producers, so there are a large number of them. Less energy is available for primary consumers, so there are fewer primary consumers, and even fewer secondary consumers, etc.

Due to the loss of energy along the food chain, it makes it very difficult for a fifth consumer (the sixth trophic level) to exist.


  1. Describe the carbon cycle.


For this one, all you really have to do is study the diagram, but for your sake, I’m writing down all the points:

Carbon is dissolved into and evaporated from large water bodies, mostly in the form of CO2.

Carbon moves into and out of the atmosphere, mainly in the form of Carbon dioxide.

Plants take carbon dioxide out of the air by photosynthesis and convert it into organic materials (carbohydrates, fats and proteins).

Herbivores eat plants, obtaining carbon compounds in the process.

Carnivores gain carbon compounds by eating other animals.

Animals and plants release carbon back into the air, in the form of carbon dioxide, through respiration.

When organisms die, they usually decompose. Decomposers break down the organic molecules through the process of respiration to gain energy, releasing carbon dioxide back into the atmosphere.

If a dead organism does not decompose, the carbon is trapped in its body. It becomes a fossil fuel over time.

Combustion of fossil fuels releases carbon dioxide back into the air.


  1. Discuss the effects of the combustion of fossil fuels and the cutting down of forests on the oxygen and carbon dioxide concentrations in the atmosphere.

Carbon dioxide is a greenhouse gas. This means it absorbs heat.

The greenhouse effect is natural and good – without it, the Earth with be a barren desert (sorta like mars), but in excess, it can have disastrous effects such as the melting of icebergs (destroying ecosystems near the poles), the rise of sea levels (causing the flooding of many coastal towns), heat strokes which can lead to death in many countries in the tropics, etc. So when there is an excess of CO2 in the atmosphere, global warming occurs, which, to put it lightly, is bad. Note that global warming is caused by the enhanced greenhouse effect.

Photosynthesis takes CO2 out of the air and replaces it with O2. Respiration takes O2 out of the air and replaces it with CO2. Therefore, photosynthesis and respiration cancel each other out, so they have little effect on the balance of CO2 in the air.

When fossil fuels are burnt, the carbon in the fuels combine with the oxygen in the air and forms carbon dioxide. This process is called combustion. This is thought to increase the carbon dioxide levels in the atmosphere.

Cutting down trees reduces the amount of photosynthesis taking place, so less CO2 is being taken out of the air, and carbon dioxide levels rise and oxygen levels fall in the atmosphere.

Therefore, the combustion of fossil fuels and the cutting down of trees have a negative effect on the atmosphere.



Notes submitted by Sarah.

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